1. Field of the Invention
This invention relates to multiple feed microwave antennas, and, more particularly, to array feeds for satellite communication system antennas.
2. Description of the Prior Art
Inadequate communications capacity is perhaps the major problem facing future satellite communication systems. Recently, satellite communication system designs have achieved a dramatic increase in the communication capacity of the satellite by discarding the single beam satellite antenna concept in favor of a multibeam approach. The modern multibeam satellite antenna permits many narrow-angle electromagnetic energy beams to be aimed at separate ground areas within a predetermined zone on the surface of a celestial body from a given aperture. In so doing, a given frequency is reused in many parts of the predetermined zone. Although a larger number of such beams can be accommodated with but a small increase in the total overall weight of the satellite, such a multibeam arrangement requires a high gain antenna. In addition, sidelobe radiation associated with each beam can cause interference between the beams. One prior art antenna which provides sufficient gain and can accommodate many feed elements on its focal surface is disclosed in detail in U.S. Pat. No. 3,914,768, which issued to the present inventor on Oct. 21, 1975.
One prior art solution to the sidelobe interference problem is disclosed in "Spectral Reuse in 12 GHz Satellite Communication Systems" by D. O. Reudink, A. S. Acampora, and Y. S. Yeh, in the Conference Record, 1977 International Conference on Communications, Volume 3, pp. 37.5-32 to 37.5-35, June 12 to 15, 1977. This reference teaches the use of plural separate narrow beams which are isolated from one another by buffer areas. The use of buffer areas, however, severely limits the number of beams which can be utilized in a given field of view, and produces areas within the predetermined zone on the surface of the celestial body which are not serviced by the satellite. Although such a spot beam approach may adequately serve nodes of heavy traffic origin, a substantial volume of traffic originating in the buffer zone area is left without access to the satellite. One prior art solution to the problem of servicing the buffer area traffic is to provide terrestrial trunking to and from the closest area served by one of the spot beams. The terrestrial backhauling approach, however, entails substantial additional costs. A second solution is to provide a wide angle area coverage antenna port, in addition to the spot beam ports, which will service the buffer areas. This approach suffers from three problems; the first being that the power requirements of this port alone might exceed the power demand of all other ports combined. Secondly, the fact that the wide angle area coverage beam also covers areas serviced by the narrow spot beams results in reception interference at all of the narrow spot beam receiver ground stations. This problem is somewhat alleviated by elaborate wide angle beam shaping techniques which selectively cancel the area coverage beam in the areas served by the narrow spot beams. By the same token, sidelobe radiation from the narrow spot beams interferes with reception at the wide angle area coverage ground stations situated in the buffer area. Finally, since the narrow angle spot beams do not overlap one another, the associated receiving stations on the surface of the celestial body must be situated within the relatively small areas illuminated by such beams, thereby rendering more difficult the tasks of determining the location of spot beam receiver sites and reconfiguring the illumination pattern of the spot beams should traffic requirements change or the satellite be reassigned to a new orbital position in space.
Another prior art solution to the problem of providing area coverage using plural spot beams is disclosed in "Design Tradeoffs for Multibeam Antennas in Communication Satellites," by W. G. Scott, H. S. Luh, and E. W. Matthews in Conference Record, 1976 International Conference on Communications, Vol. 1, pp. 4.1-4.6, June 14-16, 1976. This reference teaches the use of spot beams, each of which is launched by an associated cluster of plural feed elements. Feed elements may be shared by overlapping clusters, the respective clusters producing beams which are independent of each other because of diverse polarization direction. Thus, the shared feed elements are of a dual polarized type, and capable of down-link transmission throughout the entire electromagnetic energy frequency spectrum of the down-link frequency allocations.